The present invention relates to mass spectrometer machines and in particular to a plasma generator cavity retrofit that may be retrofit to a variety of different mass spectrometry systems.
Mass spectrometry allows atoms and molecules to be analyzed based on measured mass-to-charge ratios. The samples to be analyzed are first ionized at atmospheric pressure and then transported to a high vacuum mass analyzer through a vacuum interface. Commonly used analyzers include time-of-flight, magnetic sector, and quadrupole mass analyzers. In a time-of-flight mass analyzer the ions are accelerated in an electrical field. The amount of acceleration will be proportional to charge and inversely proportional to mass. A mass-to-charge ratio can thus be determined by measuring a time of flight of the ions (time-of-flight mass spectrometry). In a magnetic sector mass analyzer the accelerated ions are deflected in a magnetic field with the amount of deflection measured by a spatially discriminating sensor after the magnetic field. In a quadrupole mass analyzer a combination of DC and radio-frequency voltages is applied to four parallel rods. Only the ions of the correct mass-to-charge ratio satisfy the resonance condition and are able to reach the detector.
One method of ionizing samples for mass spectroscopy uses an inductively coupled plasma, for example, induced in a gas such as nitrogen or argon by a radiofrequency current passing through a conductive coil such as a copper loop surrounding the plasma. The energy of the plasma couples to the samples to ionize them.
U.S. Pat. No. 9,491,841 assigned to the assignee of the present application and hereby incorporated by reference describes an improved plasma generator using a dielectric ring in place of the copper loop. Current is induced in the ring by energy from a microwave source coupled to a radiofrequency cavity surrounding the ring.
The application of a dielectric plasma generator to mass spectroscopy is challenging because the small distance between the plasma being generated and the sampler cone of the spectroscopy machine do not provide sufficient space for the surrounding radiofrequency cavity, associated waveguides and microwave source needed to feed the cavity. In addition, the small distance makes it difficult to provide the provisions for cooling, gas exhaust, and translational adjustment.